Rubber gaskets have poor shape-retention properties when they are made of rubber alone, and are disadvantageous in that the ease of handling and the workability of mounting the rubber gaskets to desired positions are poor, thus easily causing the gaskets to fall off, or causing defective seals due to slipping.
For this reason, a layered structure of rubber and a metal core material has been heretofore produced by covering the core material with rubber so that the rubber sandwiches the core material (see, for example, Patent Documents 1 and 2).
In this case, both the rubber and the metal core must be integrated so as to prevent the core material from falling off the rubber.
The core material and rubber are typically integrated by bonding with an adhesive; this method, however, is disadvantageous in that it is costly due to the use of an adhesive.
Moreover, a siloxane adhesive is usually used as the adhesive; however, a siloxane adhesive is undesirable in that it cannot impart sufficient adhesion strength, which may result in poor contacts to devices.
Furthermore, siloxanes contained in siloxane adhesives are harmful substances, and require improvement in terms of working environment or product safety.
For this reason, as shown in FIGS. 9, 10, and 11, Patent Document 3 discloses forming a through-hole 30a in the core material 30. The through-hole 30a serves as a connecting portion 31a to prevent the rubber 31 from easily falling off the core material 30.
However, because the rubber 31 is not bonded to the core material 30, the rubber 31 may be easily peeled from the front and back surfaces of the core material 30 at regions other than the connecting portion 31a. 
That is, as shown in FIG. 11, the rubber 31 may be peeled from the core material 30, and thereby protrude out of the core material 30.
This protrusion of the rubber 31 causes the gasket to fall off the opposing seating surface to reduce the surface pressure, which may result in leakage.
Further, a technique for improving the adhesion between rubber and a metal without using an adhesive is known, as disclosed in Patent Document 4, wherein the metal surface is treated with an alkali.    Patent Document 1: Japanese Unexamined Utility Model Publication No. 2-31960    Patent Document 2: Japanese Unexamined Utility Model Publication No. 4-84862    Patent Document 3: Japanese Unexamined Patent Publication No. 2002-243042    Patent Document 4: Japanese Unexamined Patent Publication No. 11-207859